The present invention relates to an automatic braking process and, more particularly, to a method for terminating an automatic braking process for a motor vehicle, including the steps of initiating the automatic braking process upon fulfillment of a triggering criterion, building up a larger brake pressure with the automatic braking process than results from the position of a brake pedal, and terminating the automatic braking process by reduction of brake pressure to a brake pressure corresponding to a position of the brake pedal when a variable of a breaking-off criterion exceeds or drops below a threshold value assigned to the variable.
DE 40 28 290 C1 describes an automatic braking process which is to be carried out when the actuation speed exceeds a prescribed threshold value. During the automatic braking process, a higher brake pressure is built up than results from the position of the brake pedal. The automatic braking process is terminated when the pedal retraction speed exceeds an assigned threshold value. For this purpose, the brake pressure, which lies above the pressure corresponding to the position of the brake pedal, is reduced. During this process, the speed of the pressure reduction can be determined as a function of the maximum pedal travel realized by the brake pedal.
In non-prepublished Patent Application DE-P 43 25 940.5 it is proposed to match the triggering criterion to the automatic braking process. In addition, it is proposed there to terminate the automatic braking process when the force of the foot of the driver which is acting on the brake pedal drops below a threshold value.
The termination of the automatic braking process as a function of the pedal retraction speed is not reliable enough since the driver expects the automatic braking process to be terminated even with slow retraction of the brake pedal. Moreover, it is also not ensured that the automatic braking process is terminated when the vehicle is stationary.
An object of the present invention is to improve the braking process so that reliable termination of the braking process always takes place. Another object is to provide further criteria for breaking off the automatic braking process which are independent of the actuation force of the brake pedal.
These objects have been achieved according to the present invention on the basis of the automatic braking process in which a variable relating to the dynamics of vehicle movement or the activation time which has passed since the start of the automatic braking process is used as a variable of the breaking-off criterion and a gradient of the brake pressure reduction (dpB/dT) is determined as a function of at least one of the activation time (t) breaking-off criterion and an exceeding of the threshold value by the assigned variable relating to one of vehicle movement and brake pedal dependence.
Variables relating to vehicle movement dynamics can be used as the breaking-off criterion. This permits the automatic braking process to be terminated as a function of the behavior of the vehicle. In particular, the speed of the vehicle and the deceleration of the vehicle are the variables relating to vehicle movement dynamics.
A time-dependent breaking-off criterion can also be used. This time-dependent criterion ensures that the automatic braking process is terminated after a specific activation period. This criterion is therefore particularly suitable for use in conjunction with other breaking-off criteria as a safety measure in the case of operational faults. In this embodiment, it is then appropriate to prescribe a maximum operating period. According to further embodiments, the time-dependent breaking-off criterion can be determined as a function of variables relating to vehicle movement dynamics. Then, it is also suitable for use as a single breaking-off criterion. Of course, a plurality of breaking-off criteria can be used in parallel with one another or in conjunction with one another to terminate the automatic braking process. This is particularly interesting because the automatic braking process is critical for safety and it is necessary to be able to terminate it under all circumstances.
Reliably terminating the automatic braking process involves not only reliably detecting a breaking-off tinge but also reducing the brake pressure which is produced in addition to the brake pressure corresponding to the position of the brake pedal in a way which does not cause the driver to react in an undesirable way or estimate the driving situation incorrectly. Therefore, the gradient of the reduction of brake pressure can be determined as a function of the breaking-off criterion, activation time or as a function of the variable which is assigned to the threshold value of the breaking-off criterion exceeding or dropping below the threshold value.